1. Field of the Invention
The present invention relates to a technique for extracting a signal and in particular to a technique for extracting an information signal from a radio frequency signal on which the information signal is superimposed.
2. Description of the Related Art
A noncontact data carrier, such as a radio frequency identification (RFID) tag and a noncontact integrated circuit (IC) card, does not equip itself with a power supply such as a battery and instead secures the power from the energy of electromagnetic waves, which is emitted by a reader/writer apparatus for access, and performs a data communication by utilizing the electromagnetic waves.
As one example of noncontact data carriers, the RFID tag is described here. FIG. 1 is a block diagram exemplifying the configuration of a conventional RFID tag.
This RFID tag comprises an antenna unit 101, a rectification circuit 102, a capacitor C100, a shunt regulator 103, a signal extraction circuit 104, a demodulation circuit 105, a digital signal process unit 106, memory 107 and a modulation circuit 108.
A reader/writer apparatus emits an electromagnetic wave, to the antenna, by feeding a radio frequency signal on which an information signal is superimposed. Having received the electromagnetic wave, the antenna 101 outputs a radio frequency wave on which the information signal is superimposed. The output radio frequency signal is rectified by the rectification circuit 102, is charged in the capacitor C100, and then is converted into the current flowing in the digital signal process unit 106. The shunt regulator 103 performs a control so as to stabilize, at a predetermined voltage value, a voltage obtained by the rectification circuit 102 rectifying the radio frequency signal.
The signal extraction circuit 104 extracts a reception signal (i.e., the original information signal) superimposed on the signal of the power supply. The demodulation circuit 105 generates signal data on the basis of the extracted reception signal (i.e., an extraction signal ISIG). The signal data is then subjected to signal processing at the digital signal process unit 106. In this event, the digital signal process unit 106 reads or writes data from or to the memory 107 as appropriate. Meanwhile, the modulation circuit 108 modulates the impedance of the antenna unit 101 in accordance with a transmission signal generated by the digital signal process unit 106.
Next is a description of FIG. 2, which is a diagram exemplifying the circuit configurations of the antenna unit 101, rectification circuit 102 and signal extraction circuit 104, all of which are shown in FIG. 1.
An amplification-modulated, such as an amplitude shift keying (ASK) modulation, electromagnetic wave reaches at the antenna unit 101 and an antenna excitation voltage VA is generated therein. Here, the antenna unit 101 generates the input voltage VB to the rectification circuit 102 by virtue of the resistance R10 of the antenna unit 101 per se.
The rectification circuit 102, being a full-wave rectification circuit constituted by diodes D101 and D102, causes the capacitors C101 and C102 to generate a power supply voltage VDD1. Incidentally, the sign “VSS” is a grounding voltage.
Meanwhile, the antenna unit 101 causes also the signal extraction circuit 104 to generate likewise the input voltage VB. The signal extraction circuit 104 comprises a rectification circuit, which is constituted by the diodes D103 and D104 and by the capacitors C103 and C104, to generate a voltage signal VDD2 by rectifying the input voltage VB, that is, an input signal. The demodulation circuit 105 generates and outputs signal data in accordance with the generated voltage signal VDD2.
Note that a half-wave rectification circuit and a bridge rectification circuit and the like are also known as a rectification circuit, with all of the rectification circuits possessing the function of making a power supply by rectifying a voltage excited in an antenna and that of rectifying a signal and leading it to a demodulation circuit for extracting signal data.
Also note that U.S. Pat. No. 6,323,728 has disclosed a technique adding an element between a rectification circuit and a voltage control circuit controlling a power supply voltage for monitoring a post-rectification current.
With regard to the conventional noncontact data carrier, there is a problem in which an attempt to secure a stable power supply voltage makes it difficult to extract a signal.
This problem is described by taking the configuration shown in FIG. 2 as example.
The demodulation circuit 105, which extracts a signal component from an amplitude-modulated signal, such as the ASK modulation, by using signal data, demodulates a signal on the basis of the difference between an antenna-excitation voltage VA1 when the signal data of a reception signal is “0” and an antenna-excitation voltage VA2 when the signal data is “1”. Here, the definition is that the sign Ra is the value of a resistor (i.e., an impedance) R101 possessed by the antenna unit 101, the VB1 is the input voltage of the rectification circuit 102 when the signal data is “0”, the VB2 is the input voltage of the rectification circuit 102 when the signal data is “1”, the VDD1 is the power supply voltage after being rectified by the rectification circuit 102, the VDD2 is the input voltage, which is generated by the signal extraction circuit 104, of the demodulation circuit 105, the Vth is a threshold voltage common to the diodes D101, D102, D103 and D104, which are rectification elements, and the Ron is a turn-on resistance common to the diodes D101, D102, D103 and D104.
In this case, the input voltage VB1 when the signal data is “0” is represented by the following expression:VB1=VA1−(VA1−VDD1/2−Vth)*Ra/(Ra+Ron)  (1)
Likewise, the input voltage VB2 when the signal data is “1” is represented by the following expression:VB2=VA2−(VA2−VDD1/2−Vth)*Ra/(Ra+Ron)  (2)
From the above expressions (1) and (2), the difference in voltage at the input to the rectification circuit 102 between the cases of the signal data being “0” and “1” is represented by the following expression:
                                                                                          VB                  ⁢                                                                          ⁢                  1                                -                                  VB                  ⁢                                                                          ⁢                  2                                            =                                                (                                                            V                      ⁢                                                                                          ⁢                      A                      ⁢                                                                                          ⁢                      1                                        -                                          V                      ⁢                                                                                          ⁢                      A                      ⁢                                                                                          ⁢                      2                                                        )                                -                                                      (                                                                  V                        ⁢                                                                                                  ⁢                        A                        ⁢                                                                                                  ⁢                        1                                            -                                              V                        ⁢                                                                                                  ⁢                        A                        ⁢                                                                                                  ⁢                        2                                                              )                                    *                                      Ra                    /                                          (                                              Ra                        +                        Ron                                            )                                                                                                                                              =                              Δ                ⁢                                                                  ⁢                V                ⁢                                                                  ⁢                                  A                  ⁡                                      (                                          1                      -                                              Ra                        /                                                  (                                                      Ra                            +                            Ron                                                    )                                                                                      )                                                                                                          (        3        )            
Here, the ΔVA represents the voltage difference (VA1−VA2) between the VB1 and VB2.
Therefore, the voltage difference ΔVDD2 at the input to the demodulation circuit 105 between the cases of the signal data being “0” and “1” can be represented by the following expression:
                                                                        Δ                ⁢                                                                  ⁢                VDD                ⁢                                                                  ⁢                2                            =                                                (                                                            VB                      ⁢                                                                                          ⁢                      1                                        -                    Vth                                    )                                -                                  (                                                            VB                      ⁢                                                                                          ⁢                      2                                        -                    Vth                                    )                                                                                                        =                                                VB                  ⁢                                                                          ⁢                  1                                -                                  VB                  ⁢                                                                          ⁢                  2                                                                                                        =                              Δ                ⁢                                                                  ⁢                V                ⁢                                                                  ⁢                                  A                  ⁡                                      (                                          1                      -                                              Ra                        /                                                  (                                                      Ra                            +                            Ron                                                    )                                                                                      )                                                                                                          (        4        )            
That is, the voltage difference for the demodulation circuit 105 to discern between the signal data “0” and “1” is the product of coefficients greatly depending only on a voltage VA excited in the antenna and the turn-on resistance of the rectification element. Therefore, if a rectification element having a small turn-on resistance Ron is selected, the difference in amplitude at the input terminal of the demodulation circuit 105 becomes two small to discern a signal level.
On the other hand, the turn-on resistance of a rectification element needs to be minimized for effectively securing the power of a power supply and therefore a stable power supply is difficult to be secured if the turn-on resistance of the rectification element is increased by putting an emphasis on the signal detection.
Further, in the case of adopting the configuration of adding an element between the rectification circuit and voltage control circuit for monitoring an electric current as the technique disclosed in the above noted U.S. Pat. No. 6,323,728, a voltage drop corresponding to the added element occurs due to the addition of the current monitor in the power supply path, thus resulting in reducing the efficiency of rectification.
As described above, the noncontact data carrier such as an RFID tag and IC card requires an achievement of mutually incompatible functions, that is, both an obtainment of a stable power supply voltage and a firm demodulation (i.e., a signal extraction).